The present invention relates to a method for the preparation of oil-containing microcapsules and more particularly to a method for inducing a readily filterable state in the microcapsules such that the microcapsules can be easily removed from the aqueous slurry in which they are prepared and redispersed in an aqueous or oil based printing vehicle to form a coating composition.
A number of microencapsulation techniques have been used to prepare oil-containing microcapsules. Some of the principal techniques are complex coacervation (typically used to prepare gelatin capsules), in situ polymerization (typically used to prepare melamine-formaldehyde and urea-formaldehyde capsules), and interfacial polymerization (typically used to prepare polyurethane and polyurea capsules).
For some applications it is desirable to separate the microcapsules from the dispersion in which they are prepared. One such application is the preparation of coating compositions which are designed to be printed on or spot coated on paper to provide a carbonless form.
A number of techniques have been used to separate microcapsules. One of the principal techniques is spray drying. U.S. Pat. No. 4,139,392 to Davis et al. discloses a hot melt coating composition containing microcapsules in which microcapsules are spray dried to form a free flowing powder which is dispersed in a wax composition with the aid of an anionic dispersing agent.
U.S. Pat. No. 4,171,981 to Austin et al. describes another method for preparing a print on composition containing microcapsules in which an aqueous slurry of microcapsules is mixed with a hot melt suspending medium and a wiped film evaporator is used to remove the water.
U.S. Pat. No. 4,729,792 to Seitz discloses yet another method in which microcapsules are prepared by interfacial crosslinking of a polysalt formed by reaction of a polyamine and a polyanionic emulsifier with a polyisocyanate. The microcapsules are separated by adding a lipophilizing agent to the capsule slurry. The lipophilizing agent reacts with the polyanionic emulsifier and renders it non-polar such that the microcapsules precipitate from the slurry. The microcapsules can then be dispersed in an ink vehicle with the aid of a dispersing agent. It should be noted that dispersing agents are necessary for dispersing in both polar and non-polar printing ink vehicle.